The present invention relates generally to materials analysis techniques, and more specifically to material composition analysis.
Analysis of manufactured devices and materials is often required to ensure the quality of products ranging from sheet metal to semiconductor integrated circuit devices. Defects introduced during manufacturing processes can adversely affect the performance of these products. Conventionally, inspection of semiconductor integrated circuit devices by measuring the secondary electron emission levels provides adequate information concerning the structure of specific features and defects. However, semiconductor integrated circuit devices are continually getting smaller and therefore becoming more difficult to inspect. Image-based secondary electron measurement techniques become less effective for smaller samples because secondary electrons tend to scatter from the areas surrounding each feature of interest. This causes the measurements to be inaccurately representative of the feature.
To maintain high levels of inspection confidence, information in addition to structural information is required. One type of additional information is the composition of the inspected material. By using this information, defects and other imperfections can continue to be classified with confidence. Composition is conventionally determined by directing a high-energy, charged particle beam at a specimen and then measuring the characteristic x-rays that emanate from the specimen. Unfortunately however, this is very difficult since the large size, energy, costs and imprecise nature of x-ray detectors limits the accuracy of the composition measurements. Composition measurement accuracy is limited because x-rays also tend to emanate from areas that surround the areas of interest. The high energy of the charged particle beam causes the x-rays to emanate from the areas surrounding the features of interest. In view of the foregoing, a technique that can easily and accurately determine the composition of features on a sample specimen would be desirable. This desired technique would facilitate the acquisition of material inspection results at higher confidence levels.
The present invention is directed to techniques for determining the composition of a specimen under inspection by measuring backscatter electron emissions and then estimating the atomic number of the specimen. Typically, this technique is useful for inspecting features, such as defects, upon very small specimens such as semiconductor wafers. Charged particle beams are typically required to cause backscatter electrons to emanate from a specimen.
One aspect of the present invention pertains to a method that involves acquiring an image of the area of interest, scanning the area of interest with a charged particle beam such that backscatter electrons emanate from the area of interest, measuring the backscatter electrons in order to determine a total backscatter electron count, calculating a normalized backscatter electron count, and estimating the atomic number of the material within the area of interest based upon the normalized backscatter electron count, whereby the atomic number allows an operator to determine the material composition within the area of interest. One embodiment of the method further involves scanning a reference specimen having a known atomic number such that backscatter electrons emanate from the reference sample, measuring the backscatter electrons emanating from the reference specimen in order to determine a total reference backscatter electron count, calculating a normalized reference backscatter electron current, and determining the atomic number of the specimen by evaluating a relationship between the known atomic number of the reference sample, the normalized backscatter electron count, and the normalized reference backscatter electron current.
Another aspect of the present invention pertains to a computer readable medium capable of executing the operations of the described inventive method. Yet another aspect of the present invention pertains to an inspection system for performing the method of the present invention.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures, which illustrate by way of example the principles of the invention.